1. Field of the Invention
The invention concerns a salt bath composition for surface oxidation treatment of ferrous metal parts, including nitrided parts, to increase their corrosion resistance, the treatment being carried out at a temperature between 320.degree. C. and 550.degree. C., the composition including at least nitrate anions, sodium cations and where appropriate potassium alkali cations.
2. Description of the prior art
Salt baths containing alkali metal nitrates have long been used to treat ferrous metal parts, including parts that have been previously nitrided, to increase their corrosion resistance by forming a layer of magnetite Fe.sub.3 O.sub.4 to protect the underlying iron.
Document FR-A-2 463 821 describes a process for treating nitrided ferrous metal parts by immersing the parts in a molten salt bath containing sodium and potassium hydroxides with 2% to 20% by weight of nitrates of these alkali metals for a period between 15 minutes and 50 minutes. The temperatures used are between 250.degree. C. and 450.degree. C. The corrosion resistance of parts treated in this way is greatly increased compared with parts which have only been nitrided.
Document FR-A-2 525 637 describes a process of the same kind specifically intended for ferrous metal parts containing sulfur, such as parts that have been nitrided in baths containing sulfur-containing substances. The oxidizing bath contains sodium and potassium cations and nitrate and hydroxyl anions. It preferably contains carbonate anions and 0.5% to 15% of an oxygenated alkali metal salt whose oxyreduction potential relative to the hydrogen reference electrode is less than or equal to -1 volt, such as a bichromate. An oxygenated gas is blown into the bath and the concentration of insoluble particles in the bath is maintained at less than 3% by weight. This produces good corrosion resistance (250 hours in the salt spray test) without deterioration of wear and fatigue resistance and there is an improvement in seizing resistance under conditions of dry rubbing.
However, it has been found that this performance cannot be achieved with the reliability and reproducibility required to meet industrial demands. Performance variations are relatively minor in the laboratory. They become much greater for treatment carried out on an industrial scale. They are particularly noticeable when large quantities of small parts are "bulk" treated or parts with imperfect surfaces are treated: the presence of disrupted areas such as pressing or punching burrs, crimping or bending creases and welding heterogeneities are all sources of defects and therefore of corrosion.
A random resistance to corrosion is totally unacceptable for parts such as jack or damper piston rods and automobile windshield wiper and starter motor spindles. The solution to this problem has for many years been to refresh the baths repeatedly, as and when required, according to the more or less aberrant results obtained. This solution is not satisfactory, in particular because of the industrial requirements mentioned previously.
The proportions of the bath constituents (hydroxides, carbonates, nitrate, bichromate) have been varied to improve reliability and corrosion resistance. Our investigations have shown that to achieve excellent corrosion resistance (i.e. more than 400 hours exposure to salt spray before the first appearance of traces of corrosion), the surface of the parts must be a uniform deep black color, typical of the formation of a layer of magnetite Fe.sub.3 O.sub.4 with good crystalline order. At the same time, the corrosion potential in a 30 g/l NaCl solution relative to a saturated calomel electrode should be 1 000 mV to 1 300 mV, indicative of complete passivation.
The correlation between the oxyreduction potential of the oxygenated salt (e.g. bichromate) and the desirable corrosion potential should be noted.
However, baths containing alkali metal hydroxides, nitrates, carbonates and bichromate or permanganate require frequent testing of the bath composition and adjustment to the operating conditions specific to the parts if efficiency is to be maintained. Also, performance varies due to modification of the composition of the bath by consumption of reagents, soiling by residues on the parts due to previous treatments and reaction of the soiling materials with the bath constituents, entrainment of bath constituents with parts removed from the bath, and reaction of the hydroxides in the bath with carbon dioxide in the atmosphere; these performance variations occur despite periodic adjustment of the bath composition. In specific applications the strong oxidizing agent (bichromate) concentration is relatively critical.
Enrichment of the bath with carbonates due to oxidation of nitriding bath cyanates and absorption of carbon dioxide from the atmosphere lead to precipitation of carbonates that form a sludge at the bottom of the bath. Removal of this sludge entrains active constituents of the bath.
The invention concerns oxidizing bath compositions based on alkaline-earth metal nitrates which have a reliable and repetitive oxidizing power.